Make category filtering a reusable function for live filtering

Can now be used anywhere where a "real-time" filter is desirable.
As a usage example, it's implemented for the list of installable
plugins. Should work anywhere, though – needs an input element,
a list of target elements and an element within those target
elements to contain the search term.

Also made the function for the reset button for this reusable.

References #154

docs/NEWS

@@ -4,6 +4,10 @@
 Version 2.0-beta3 ()
 ------------------------------------------------------------------------
 
+    * Changed JS for category filtering and its reset button to be a
+      reusable function, which is now also used in the list of
+      installable plugins.
+
     * Fixed wrong local documentation URL in plugin configuration
 
     * Added new "backend_dashboard" event-hook for plugins to use